In the past two quarters, Android phones' share of the U.S. smartphone market jumped from four percent to 13 percent. Even more impressively, in the last six months Android grabbed 27 percent of all new smartphone subscribers — more than the iPhone (23%) and closing in on flagging leader BlackBerry (33%), both of which saw new subscriptions fall.

Those numbers being startling, and this being a blog entry, we must now immediately declare What This Means: Apple is dying! Google is God! BlackBerry is irrelevant!

Not really, of course. The numbers tell a far more complex story than that.

To start, smartphone sales are now a quarter of the U.S. mobile market according to Nielsen, which predicts they'll outsell other kinds of phones by the end of 2011. The Times'Bits blog reports on another study that puts smartphone growth at 64 percent in the second quarter of 2010 alone. A rising tide lifts all ships; it's just that some ships are lighter than others. Easier for Android to go from nothing to something (their share grew at 886 percent in that same period) than for Apple to go from dominant to more dominant.

What's more, when Nielsen asked iPhone users what their next likely phone would be, 18 out of 20 said another iPhone, while only one out of 20 said they wanted to switch to Android. Only 14 out of 20 Android users said they would stay with Android, while four out of 20 said they would switch to the iPhone. (BlackBerry owners are most torn, with 8 out of 20 sticking with their little keyboards, six out of 20 planning to move to an iPhone and four of 20 looking for an Android).

On the Bits blog, one commenter noted that "it is not entirely reasonable to compare Android, which is an OS to iPhone, which is a phone. iPhone is a single device, provided by a single carrier while there are, as I believe, tens of smartphones running Android and they are offered by each and every US carrier."

But in fact that's precisely why it's compelling to compare the two. The Nielsen study compares phone operating systems, but since the iPhone OS appears nowhere besides its own device with a single carrier — Apple's strategic decision — there's no choice but to compare many Android OS devices to one iPhone OS device. Apple's strategy created this reality.

Apple, as ever, is closed. Our device. Our OS. Our app store with our rules. Hell, you can't even get at the iPhone's battery. Android is more open, with several carriers and hardware options, fewer app restrictions and even Flash support.

Still, Android hasn't yet earned the loyalty of the iPhone. It's off to a strong start, and making some"lustworthy" devices, sure, but maybe Apple will open up the iPhone. Or maybe openness will turn out to be less important on mobile devices than it was on desktop computers. Also, Apple's doing pretty well so far with the same closed strategy on that tablet thing, too. On the other hand,a slew of Android tablets are on their way.

Apple's closed approach has worked remarkably well so far — though recently some scratches have shown up on Apple's shiny touchscreen image. And the last time Apple fought an open-versus-closed OS battle, it got demolished by Windows. Few people ever claimed Windows was superior technology, but its strategy was superior, and market share reflected that. The startling rise in Android adoption could well reflect the same consumer desire for an open platform. For choice. For competition.

That's what I take away from these new market share numbers on the iPhone and Android (oh yeah, and BlackBerry, too). Finally, we've got a good fight on our hands. And when companies have to actually fight for their customers, guess who wins.